Being somewhat familiar with Zestiria, I think we can maybe come to a workable compromise! Because the ability to see seraphs is based on a resonance and stuff like sensing evil, this would be more in like with a magical ability which would fluctuate based on world frequency. Stuff like not needing to eat or sleep is a bit more strictly physical based on their status as seraphic creatures.

In the case of the latter, some later events may make these physical non-necessities more null and void, so you may have the ability to play around with these a little bit more in the future. If you think this is more workable, let me know! It's expected to need to give some characters a little wiggle room in order to make the game experience workable and engaging for players.